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I want to know if the following has been done experimentally; after the spin (or any other characteristic with a probability of 50%) of 2 entangled particles has been measured, we change the spin of one and we see the spin of the other changing instantaneously at a distance.

For example, entangled particle A is spinning up and entangled particle B is spinning down, we make A spin down and see B start spinning up at the same time.

I know entanglement has been proven experimentally but it always seems to imply this "spooky action at a distance" and I wonder if THAT has actually been proven experimentally. Maybe my question should have been has anyone seen spooky action at a distance...

David Z
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user6328
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2 Answers2

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No. What you describe is not what is meant by quantum entanglement. What you describe would allow instantaneous communication across large distances which would allow violations of causality and would violate special relativity.

Quantum entanglement occurs when you prepare two particles such that one is spin up and the other is spin down, but you don't know which is which. You let these two particles travel (at less than or equal to the speed of light) to two measuring apparatus that can each be set at any angle, not just up/down, and measure the spins. What you find, when you compare both measurements is that the results are correlated as expected by quantum theory. For example, if both apparatus measure at 45 degrees to the prepared spin axis, the measured spins will always still be equal and opposite. See this for a full explanation.

There is no communication at all between the particles once they are separated. Thus, entanglement cannot be used for faster than light communication since it is only the comparison of results that is predicted.

Update: Another great explanation of entanglement is here.

FrankH
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  • Thanks a lot that's what I thought it was but I read this from Brian Clegg (has a degree in physics from Cambridge University) "It’s possible to link together two quantum particles — photons of light or atoms, for example — in a special way that makes them effectively two parts of the same entity. You can then separate them as far as you like, and a change in one is instantly reflected in the other. This odd, faster than light link, is a fundamental aspect of quantum science." and I got confused... – user6328 Nov 24 '11 at 14:01
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    You are welcome. It is a common misapprehension about entanglement. Many science fiction stories also claim that entanglement allows faster than light communication, but it just ain't so. I guess that is why it is called fiction. – FrankH Nov 24 '11 at 14:09
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    Maybe you should clarify the Wikipedia entry "Experimental results have demonstrated that effects due to entanglement travel at least thousands of times faster than the speed of light.[20][21] In another experiment, the measurements of the entangled particles were made in moving, relativistic reference frames in which each respective measurement occurred before the other, and the measurement results remained correlated." – Fraggle Nov 24 '11 at 14:48
  • @Fraggle Thanks for pointing wikipedia thing out. It looks like some kind of fraud. – Pratik Deoghare Nov 24 '11 at 16:29
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    There is nothing wrong with the wiki description. It really tells the same as FrankH, it does_not_suggest that information was send thousands of times faster than the speed of ligth. The effect(which is theoretically instantaneously) was measured experimentally to be at least this speed. – hpekristiansen Nov 24 '11 at 20:54
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    @Fraggle, Thanks. Wikipedia is not exactly wrong, but it is misleading. So I edited as follows: "Experimental results have demonstrated that effects due to entanglement travel at least thousands of times faster than the speed of light....[22][23] Although the "effect" of quantum entanglement appears to exceed the speed of light, there is no violation of special relativity or causality which declares that information cannot be transferred faster than the speed of light. See EPR Paradox for an explanation about how causality is not violated." – FrankH Nov 24 '11 at 20:58
  • @FrankH I think this answer is somewhat misleading as it seems to suggest a hidden variable explanation to the correlation. –  Nov 25 '11 at 04:23
  • @zephyr - sorry, I don't see how I am suggesting hidden variables anywhere - did you mean in my answer to the question or my edit of wikipedia? I would be happy to edit anything I said that suggests hidden variables. – FrankH Nov 25 '11 at 04:32
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    I sort of wonder if the word "effect" is appropriate at all here. – Fraggle Nov 25 '11 at 05:17
  • @fraggle - 1st the word "effect" is only in the wikipedia article not in my answer to this question. I personally don't see how "effect" implies hidden variables, but in any case that word was already used several times in the wikipedia article, so I suggest you take this issue up on that page and make whatever edit you think is more appropriate on the wikipedia page for Quantum Entanglement. – FrankH Nov 25 '11 at 05:52
  • @FrankH - The part I disliked is this: "Quantum entanglement occurs when you prepare two particles such that one is spin up and the other is spin down, but you don't know which is which." From the rest of your answer I don't doubt that you understand entanglement, I just feel like that sentence may give people the wrong idea (that instantaneous communication does not occur because the particles have a hidden shared property) –  Nov 25 '11 at 17:35
  • @zephyr - Ok. I kind of see your point, but it is very subtle and I cannot think of how else to say that without someone reading hidden variables into it. Any suggestion on wording? – FrankH Nov 25 '11 at 18:22
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    Why not say that there's is no communication between the particles at all after they've been separated. The effect only occurs because before we measure any of the particles we don't know what state their in. Back to the wiki article; saying that the effect "travels at least thousands of times faster than the speed of light" is what I find wrong, the effect "remains after separation regardless the distance" would be better wording. What do you think? – user6328 Nov 27 '11 at 10:24
  • @user6328 - I added part of what you suggested here. Thanks for the suggestion. I did not add the second part you suggested since I didn't think it really added any new information. About the wiki article, feel free to make whatever edits you think are appropriate. I do like your suggestion of not even mentioning speed in the wiki article. – FrankH Nov 27 '11 at 13:39
  • I didn't write it, but I think the reason Wiki says the effect travels thousands of times faster than light is because this must be true in a hidden variables theory. In pure QM, nothing travels at all, because its just nonlocal entanglement, without any hidden variables. But the bound on speed makes it clear that any hidden variable theory has to be grossly nonlocal. Perhaps the best phrasing is "If the entangled particles have hidden variables which determine the outcome, entanglement requires that these variables change at least 10000 times faster than light in response to measurement". – Ron Maimon Dec 03 '11 at 09:32
  • @RonMaimon : it's a new miscomprehension of the datas todays, in this great grants war. The process claimed ( naturally ) by the localists and causelists has no speed, since it is a causal result. Only non-causelists non-localists need to define a speed for the famous effect. Hence, there is no new constraints on hidden variables theories. We still wait that the nonlocalists produce the promised technologies since they are so sure to have their proofs. 25 years is a lot. –  Jan 30 '16 at 12:50
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Do you mean literally see it? I guess they are working on experiments like that: http://www.wired.com/wiredscience/2010/06/human-quantum-entanglement-detector/

Or do you mean it is it physically real? This says it is: http://www.nature.com/news/quantum-theorem-shakes-foundations-1.9392

Joe
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  • Ok those articles are confusing because again they imply a "spooky action at a distance". The way I see entanglement is like this; imagine the 2 particles are 2 aces from a deck of cards, they become entangled in such a way that if one is of hearts the other must be spades (lets pretend there's only 2 suits) If I scramble them and give one to you and keep the other it doesn't matter how far apart we are if I look at my card and it's ace of spades then I know yours is the ace of hearts without having to call you to find out. No communication needed, no spooky action between the cards etc... – user6328 Nov 29 '11 at 15:52